Encapsed non-literal non-empty/falsy strings always return generic string

[kkmuffme]

https://psalm.dev/r/3dfe7f61d3

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/3dfe7f61d3

<?php

/**
 * @var non-falsy-string $a
 * @var non-falsy-string $b
 */
$x = "{$a}{$b}";
/** @psalm-trace $x */;

Psalm output (using commit 08afc45):

INFO: Trace - 8:23 - $x: string

INFO: UnusedVariable - 7:1 - $x is never referenced or the value is not used